1. Field of the Invention
The present invention concerns a rolling bearing device typically represented by linear motion guide units, as well as rolling bearings, ball screws, water pump bearings, uniform speed joint races and hub unit bearings for use in automobiles and, more in particular, it relates to an induction-hardened rolling bearing device which is excellent in wear resistance, has long service life, as well as excellent in cold drawability and capable of extending the serving life of a die by induction hardening.
2. Statement of the Prior Art
Heretofore, a technique of applying induction hardening to parts made of carbon steels for use in mechanical structures, among all, medium carbon steels (C: 0.40 to 0.60%) as typically represented, for example, by automobile parts to increase the hardness only at the surface of them and improve static bending/twisting strength or rolling contact strength. For such application uses, steel materials such as JIS-S53C steels have been mainly used. For example, Japanese Patent Unexamined Publication Hei 6-57324 discloses a method of manufacturing an induction-hardened bearing for use in automobile hub units containing, as main ingredients, from 0.50 to 0.65% of C, from 0.07 to 0.15% of Si, from 0.05 to 0.35% of Mn, from 0.25 to 0.55% of Cr, and 0.0035% or less of B by weight ratio as a material for at least one of an outer ring and an inner ring as bearing parts to be applied with drilling fabrication.
Further, Japanese Patent Unexamined Publication Hei 5-59486 discloses, as another technique, steels used for cold forging, excellent in induction-hardenability and fatigue characteristic by forming a material containing from 0.4 to 0.6% of C, 0.1% or less of Si, from 0.2 to 0.4% of Mn, 0.015% or less of P, from 0.005 to 0.015% of S, from 0.20 to 0.50% of Cr, from 0.08 to 0.30% of Mo, from 0.0005 to 0.0030% of B, from 0.02 to 0.05% of Ti, from 0.01 to 0.05% of Al, 0.006% or less of N and 0.002% or less of O, in which the sum of Cr and Mo is from 0.30 to 0.80%.
Further, Japanese Patent Unexamined Publication Hei 6-341432 discloses a linear motion guide unit having a hardness Hv of a induction-hardened raceway track member of a guide rail increase to 700 or more and improved with wear resistance, without deteriorating the cold drawability by defining the composition of the steels as from 0.56 to 1.0% of C, 0.15% or less of Si, from 0.20 to 0.40% of Mn, from 0.30 to 0.70% of Cr and from 0.0005 to 0.0035% of B.
Generally, hardness of steel material after induction hardening and tempering becomes higher as the carbon content in the steel materials increases. In steel materials such as JIS-S53C described above, a hardness of about HRC 59 can be obtained as the surface hardness by properly controlling the heat treatment, but rolling fatigue resistant life and wear resistant characteristic are still insufficient. Further, there is also a problem that the cold workability of the steel material is lowered as the carbon content increases and this leads to the increase in the die cost in the cold drawing process. If the drawing speed is lowered with an aim of reducing the cost, the productivity is worsened.
For the bearing used for hub units disclosed in Japanese Patent Unexamined Publication Hei 6-57324, since the compositional range of ingredients in the alloy such as C, Si, Mn and Cr is defined, it has a remarkable advantage that the maximum value for the hardness Hv does not exceeds 230 even if the annealing step after hot forging is saved, so that drilling fabrication can be applied without deteriorating the tool, as well as that a hardness suitable to practical use can be ensured for raceway track members. However, a problem exists regarding the rolling fatigue characteristic. That is, as shown in FIG. 5, the rolling raceway surfaces 2 of an induction-hardened hub unit (cross-hatched portion) tend to suffer from abrasion or dints deleterious to bearing life, due to the lowering of a lubricant viscosity caused by intrusion of muddy water or obstacles, and there is still a room for improving such drawbacks.
Further, the steels for use in cold forging disclosed in Japanese Patent Unexamined Publication Hei 5-59486 are excellent in the hardenability but most of Ti merely added by from 0.02 to 0.05% is exhausted for fixing N required for incorporating B and no sufficient effect can be expected for the wear resistance. It rather gives rise to a problem of increasing the possibility for the presence of Ti system non-metal inclusions having a size of several tens of micrometers which are deleterious to the rolling fatigue life.
Further, the linear motion guide unit disclosed in the Japanese Patent Unexamined Publication Hei 6-341432 is satisfactory regarding the cold drawability and also has satisfactory rolling fatigue characteristic compared with conventional steels. However, while the wear resistance under the current working circumstances is sufficient, there is still a room for the improvement regarding the rolling fatigue characteristic and the wear resistant characteristic when working circumstances are worsened in the feature.